The present invention relates in general to laser and nonlinear optical equipment, and more particularly to laser equipment in which the energy of an input fundamental wavelength is converted into a different output wavelength.
Laser systems have been widely utilized in the medical field for the treatment of tissue. The high intensity energy of a laser beam can be concentrated into a small cross-sectional area and used with different types of tissues to accomplish different functions, such as cutting, cauterizing, cell destruction, etc. It is well known that laser systems operating with various fundamental wavelengths are advantageous for different types of operations. For example, in ophthalmic surgical operations, it has been found that a YAG-type laser generating a wavelength of 1.064 or 1.32 nanometer (nm) is especially advantageous for cyclophotocoagulation. It can be appreciated that in other medical-surgical applications, certain wavelengths of laser beams are more advantageous than others.
The various well-known laser mediums, such as crystals, semiconductors, gases and dyes, each generate a characteristic fundamental wavelength of coherent radiation. While there are available a host of lasing mediums available for generating a corresponding number of laser radiation beams having desired fundamental wavelengths, there exists certain wavelengths which may be desirable to generate, but in which there currently exists no lasing medium for generating the desire fundamental wavelength. In addition, many businesses specializing in the design and development of laser systems also specialize in one or a few particular types of mediums, such as crystals or semiconductors, etc., and do not develop an expertise in working with other types of mediums, such as gases and dyes. Accordingly, it often occurs that laser radiation beams of certain wavelengths are desirable, but are unavailable because of the particular mediums with which the developer is familiar.
One technique for generating an output laser radiation beam having a different wavelength than that generated by the laser medium is by the use of wavelength doubling or tripling crystals. Specialized harmonic crystals have been developed for use with currently available laser mediums to provide an output wavelength different from the characteristic wavelength generated by the lasing medium itself. Disclosed in U.S. Pat. Nos. 3,949,323 (DuPont) and 4,826,283 are techniques for fabricating a harmonic crystal for use with lasing mediums, where the crystal is responsive to an input fundamental wavelength to produce an output harmonic wavelength. The disclosure of these patent is incorporated herein by reference. Crystals adapted for generating harmonic wavelengths include the following types: Potassium dihydrogen phosphate (KDP or KH.sub.2 PO.sub.4), Potassium dideuterium phosphate (KD*P or KD.sub.2 PO.sub.4), Potassium titanyl phosphate (KTP or KTiOPO.sub.4), Lithium triborate (LBO or LiB.sub.3 O.sub.5), Beta-barium borate (BBO), KTA, lithium niobate doped with MgO, Lithium iodate (LiIO.sub.3), MgO:LiNbo.sub.3, RbTP, RbTA, YAB, KNbO.sub.3, Urea, BANANA crystals, and others. Such type of crystals are generally grown and cut along certain axes which are phase matched according to the harmonic wavelength desired, and thus provide maximum efficiency at such wavelength. These crystals can be pumped or otherwise energized with the fundamental wavelength produced by a lasing medium, whereupon the desired harmonic wavelength of radiation is emitted. A more complete discussion of nonlinear devices and crystals used in such devices can be found in W. Koechner, Solid-State Laser Engineering (2d ed. 1988), which is incorporated herein by reference.
An application of a laser system generating fundamental/harmonic wavelengths is illustrated in U.S. Pat. No. 4,907,235, the entire contents of which are incorporated herein by reference. FIG. 13 of the noted patent illustrates a system using a nonlinear crystal between the frontal mirror and the lasing medium. While such a system appears to be effective in producing an output harmonic wavelength, any harmonic wavelengths reflected internally by the removal means can be absorbed by the lasing medium, thereby losing this laser energy and unnecessarily increase the temperature of the lasing medium.
This patent, in FIGS. 9, 10, 11 and 12, also illustrates a linear system, as opposed to the folded systems illustrated in FIGS. 8, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 and 25. However, the illustrated linear systems do not produce a harmonic output at the end of the system. Rather, the harmonic output is produced along an axis that intersects the longitudinal axis of the lasing medium and requires an angled harmonic removal medium. Angling the removal medium polarizes the fundamental radiation output of the lasing medium thereby reducing the corresponding harmonic output of the system.
From the foregoing, it can be seen that a need exists for an improved linear cavity adapted for generating a harmonic wavelength and in which the harmonic wavelength internal to the cavity is substantially prevented from being reflected to or reaching the lasing medium.